Permanent magnets assembly means for correction of pincushion distortion in cathode ray tubes



April 2, 1968 c. A. FRANKLIN 3,376,450

1 PERMANENT MAGNETS ASSEMBLY MEANS FOR CORRECTION OF PINCUSHION DISTORTION IN CATHODE RAY TUBES [NYE/V702.

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Filed Aug. 24,

3,376,450 PERMANENT MAGNETS ASSEMBLY MEANS FQR CORRECTION OF PINCUSHION DISTORTION IN CA'li-IODE RAY TUBES Chester A. Franklin, North Hollywood, Calif., assignor to International Telephone and Telegraph Corporation, New York, N.Y., a corporation of Maryland Filed Aug. 24, 1966, Ser. No. 574,782 9 Claims. (Cl. 313-434) This invention relates to cathode ray tubes and similar devices, and more particularly to an improved means for mounting permanent magnets utilized to correct or modify the shape of the picture formed by the tube electron beam output.

In cathode ray tubes, the electron beam is typically bent or deflected by electromagnetic means positioned in a yoke surrounding the tube to form a desired geometric shape on the raster or face of the tube. Commonly, the beam is deflected into a rectangular pattern which due to the relatively fiat screen or face of the tube and the nature of the deflecting means is normally distorted around the corners of the shape. More specifically, these factors typically cause the sides of the rectangule to bulge inwardly which in turn causes the corners to appear as if they are distorted outwardly. This condition is commonly referred to in the trade as the pincushion effect named after the bulging appearance of the fields produced by the electromagnetic deflection means.

The usual method to correct or modify the shape of the picture to eliminate this pincushion effect is to employ additional magnetic means located near the corners of the pattern to force the corners inwardly to be more aligned with the remainder of the shape perimeter. In one approach, four permanet magnets are mounted around the cathode-ray tube, usually near the neck portion, with each magnet being positioned adjacent to a corner of the rectangle projected onto the face of the tube. Due to the various tolerances and other variables in such apparatus, it is often desirable to individually adjust each permanent magnet for each tube to obtain the desired shape.

It is the principal object of this invention to provide an improved simplified means for mounting these permanent magnets in a manner to readily permit the necessary adjusting.

In accordance with this invention, there is provided an annular plate to be placed around the neck of a cathoderay tube carrying a pair of spaced spring retainers, and a number of retaining elements corresponding to the number of magnets to be utilized, are secured to the spring retainers so that the magnets may be resiliently and frictionally clamped between the elements and the mounting plate. With such arrangement, the permanent magnets may be moved in any planar direction on the plate beneath the retaining elements. Also, the springs and retaining elements may be circumferentially moved to permit selection of the desired geometric shape.

In one form of the invention, the spring retainers comprise an inner wire attached to the inner edge of the annular mounting plate and an outer wire attached to the outer edge of the mounting plate. Each wire is formed with axially projecting loop portions with the portions on the inner wire being radially aligned with the portion on the outer wire. The retaining elements extend between the aligned loop portions to thus secure the magnet. As can be seen, this approach permits the retaining elements to be circumferentially moved or slid on the wires. Such a construction not only provides all of the desired and necessary adjustment means but in addition is very inexpensive and easy to assemble.

In prior art arrangements employing permanent mag- 3,37,45b- Patented Apr. 2, 1968 nets, the magnets have either been permanently fixed without any adjustment means or have been adjustable in a limited fashion by loosening screws with a screw driver and so forth. In another prior art app-roach, specially fabricated arcuate permanent magnets are mounted in an annular track and hence are circumferen'tially adjustable but not radially nor rotationally about the axis of the magnet.

Electromagnets are also utilized to correct the shape of the cathode ray picture, however, the magnets or coils producing the magnetic fields are permanently fixed or potted in insulation. Consequently, although the strength of the magnetic field may be varied by varying the amount of power applied to the coils, which corresponds to radial adjustment of permanent magnets, no circumferential or polarity adjustment is provided.

For a better understanding of the present invention and a further appreciation of its advantages, refer to the following detailed description and drawing in which:

FIG. 1 is a diagrammatic showing of a cathode-ray tube having the corrector assembly of this invention mounted thereon;

FIG. 2 is a perspective view of the corrector assembly;

'FIG. 3 is a bottom plan view of the assembly of FIG. 2;

FIG. 4 is a cross-sectional view along 44 of FIG. 3;

FIG. 5 is a cross-sectional view along 5-5 of FIG. 3;

FIG. 6 is a diagrammatic illustration of the corrector assembly with the permanent magnets positioned to outline a rectangle; and

FIG. 7 is a diagrammatic showing of the assembly with the permanent magnets outlining a square.

Referring now to FIG. 1, the cathode ray tube 10 represents any vacuum tube device in which a stream of electrons are directed to form a beam and such beam is deflected to move in a desired pattern or image which appears on a phospherous screen on the front of the tube. On the neck 11 of the tube there is mounted a core 12 containing a pair of vertically oriented magnetic coils and a pair of horizontally oriented coils (not shown) which are employed to deflect or bend the beam of electrons in a desired manner. As explained above, the rectangular pattern or image normally formed on the screen of such tubes is commonly distorted around the corners of the image due to the geometry of the screen with relation to the electron source and due to the nature of the deflecting means within the core 12. In this situation, the corners of the rectangle appear distorted outwardly while the mid-sections of the boundaries of the rectangle bulge in wardly.

A corrector assembly 14 mounted on the tube neck 11 in front of the core 12 is utilized to modify or correct the shape of the image such that the boundaries form straight lines. In a well-known manner, the corrector assembly employs a plurality of permanent magnets which are approximately aligned with the corners of the picture on the screen to bend or deflect the beam of electrons more correctly in line with the mid-section of the side-walls of the picture. The magnets are of the simple bar type which are mass produced and available as an cit the shelf item and thus are relatively inexpensive.

Due to manufacturing tolerances in the magnets, in the cathode ray tube, and in the yoke deflecting means, it is desirable that each magnet be individually adjustable to most accurately correct the shape of the picture. In accordance with the invention, the corrector assembly incorporates means providing a simple and inexpensive method for mounting the magnets to attain this desired adjustability.

The primary supporting member of the corrector assembly is an annular plate 18 which may be conveniently molded of plastic or similar material. The lower side of the mounting plate 18 is formed with an annular series of depending lugs or fingers 20 which cooperate with an open end of the container enclosing yoke 12. Note that to permit proper alignment of the mounting plate with respect to the yoke and the cathode ray tube, the plate may be freely rotated. The inner edge of plate. 18 is formed with a plurality of radially extending projections 22 which are uniformly spaced to define a plurality of notches or spaces 24. Similarly, the outer edge of plate 18 is formed with a series of projections 26 defining a series of nottches 28.

In accordance with the invention, a uniquely formed pair of wires 30 and 32 are secured to the plate in cooperation with the notches and projections. More specifically, the outer retainer wire 30 is formed with a diameter approximately equal to or slightly less than the outer diameter of plate 18 while inner retainer wire 32 has a diameter slightly greater than the inner diameter of plate 18. Retainer wire 30 is positioned in a groove on the under side of the mounting plate 18 and ecured thereto by fasteners 34 adjacent to the outer edge while wire 32' is similarly mounted adjacent to the inner edge of the plate.

As best seen in FIG. 2, each wire 30 and 32 is formed with four axially projecting looped portions 30a and 32a respectively. Looped portions 30a are located so that they extend through notches 28 between projections 26 to the top side of plate 18 where they are spaced from the top surface of the plate. In like fashion, the looped portions. 32a of retainer wire 32 extend from the bottom side of plate 18 upwardly through the notches 24 between projections 22 to the top side of the plate where they are spaced from the top surface of the plate. As seen, the looped portions 30a are radially aligned with looped portions 32a.

Four retaining clips 36 having hooked end portions 36a and 36b are secured to the looped portions 30a and 32a. That is, hooked portion 36a is secured to looped portion 30a while hooked portion 36b is secured to looped portion 32a. Thus, the clips 36 are spaced from the upper surface of the mounting plate and thereby define a space in which the magnets 16 may be clamped. As best seen in FIG. 4, the magnets 16 are snugly and frictionally retained between the clips and the mounting plate. The resiliency of the wires 30 and 32 acting on the clips 36 hold the magnets in this position if the thickness of the magnet is slightly greater than the space between the clip and the mounting plate.

It can be seen that with such an arrangement, the magnets may be slid beneath the clips in any direction in the plane of the mounting plate. Thus, the position of each magnet may be individually adjusted by simply moving the magnet beneath the clip 36. In this fashion. the effect on the cathode ray electron beam may be suitably modified or corrected. By radially moving the magnets, the strength of the magnetic field as applied to the cathode ray tube is varied. By moving the magnets circumferentially, the magnet may be properly aligned with respect to the diagonal of the picture formed on the screen. It should be noted that the magnet cannot only be moved circumferentially beneath the clip, but the entire clip may be circumferentially moved or slid on the retaining wires. Finally, the magnets may be rotated or twisted beneath the clip to affect the polarity of the field applied to the cathode ray tube. Thus, it can be seen that this simple mounting means provides easy adjustment, in any desired manner, of the magnetic field applied to the tube.

It should be pointed out that no critical tolerances are involved in the mounting arrangement in that the wires can be bent a certain amount as well as the Clips 36. Accordingly, standard magnets of a variety of sizes and shape may be employed with little difficulty. Once the desired location for each magnet is obtained, a drop of adhesive may be used to insure that the magnet does not inadvertently move from its adjusted position.

As another feature of this invention, the circumferential ends of the wire looped portions 30a and 32a are located to mark the boundaries of desired geometric shapes. Most often, the picture on a cathode ray tube is either to be square or rectangular with a three to four ratio between adjacent sides. Accordingly, the circumferential ends of the looped portions have been positioned and formed sothat the clips may be moved between two extreme positions on their respective looped portions to define either of these shapes. In FIG. 6, the clips 36 are shown positioned on the wires abutting the circumferential ends of the looped portions 30a and 32a near a horizontal diameter through the plate or the clips as close as possible to the horizontal diameter and as far as possible from a vertical diameter through the plate. In this position, the clips and hence the magnets beneath them define the rectangular shape having the three by four ratio.

By contrast, the clips have been moved to the opposite extreme on their respective looped portions in FIG. 7. There it can be seen that the clips have been moved as close as possible to the vertical diameter and as far away from as possible the horizontal diameter. Actually, in FIG. 7, the clips are equally spaced from the horizontal and vertical diameters so that they essentially define a square. Thus, it can be seen that by simply sliding the clips between these extreme positions the corrector shape can be readily changed. Naturally, with this versatility, the single corrector assembly is more convenient and savings may be obtained in manufacturing.

While the basic utility of the corrector assembly is as explained to correct or modify rectangular shaped images on cathode ray tube screens, it should naturally be understood that the electron beam may be modified in some manner or for some purpose other than to correct the so called pincushion effect. For any such other function, additional magnets and looped portions on the wire retainers may certainly be employed and the looped portions may be varied in circumferential dimensions or axial dimensions to provide the desired result- Similarly, other various modifications and changes may come to mind in light of the foregoing disclosure; and it is therefore intended that all such changes and modifications that fall within the true spirit and scope of the invention be included in the appended claims.

What is claimed is:

1. In a vacuum tube producing a beam of electrons which is deflected in a manner to form a desired image on a fluorescent screen, a device for modifying the geometric shape of the image comprising: a mounting plate for surrounding a portion of the tube through which the beam passes; a pair of spaced spring retainers attached to said plate; a retaining element attached to and extending between said spring retainers; a permanent magnet clamped between said element and said plate as urged by said spring retainers, the dimensional relation of said element and retainers being such as to permit said magnet to be positioned in all directions in a plane parallel to said plate and to permit said magnet to be rotated on its own axis.

2. The device of claim 1 wherein said spring retainers comprise a pair of spaced wires having portions spaced from said plate and said retaining element is attached to said spaced portions with the magnet positioned between the element and the plate.

3. In a device employing permanent magnets for modifying the shape of the output from a cathode ray tube to modify the shape of the image produced on the raster of the tube, improved means for mounting said magnets comprising: an annular plate surrounding the neck of said tube; spring means attached to said plate; a plurality of magnet retaining elements circumferentially spaced about said plate by said spring means; said elements being held spaced from said plate 'by said spring means to define a space between each element and said plate for frictionally receiving a magnet in each space, said retaining element and said springs permitting movement of said magnets in all directions on said plate while held to said plate.

4. The construction of claim 3 wherein said spring means includes an inner annular wire having a diameter approximately equal to the plate inner diameter and being secured to the plate inner edge, and an outer annular wire having a diameter approximately equal to the plate outer diameter and being secured to the plate outer edge, each of said wires having circumferentially spaced, axially extending, and radially aligned loop portions, each of said retaining elements being attached to a radially aligned pair of said loop portions.

5. The mounting means of claim 4 wherein said retaining elements are radially extending clips having hooked end portions with one end portion being hooked onto a loop portion of said inner wire and the other end portion being hooked onto a loop portion of said outer wire.

6. The mounting means of claim 5 wherein said loop portions extend arcuately a considerable span to permit said clip to be circumferentially moved on said loop portions.

8. The mounting means of claim 6 wherein said wires include four radially aligned pairs of loop portions for receiving four of said clips, said clips being movable to one position engaging one set of ends of said loop portions which define a square, and said clips having another position wherein each clip engages the opposite end of its loop portion to define a rectangle having a predetermined length to width ratio.

9. The mounting means of claim 4 wherein the inner and outer edges of said plate are formed with a series of spaced radial projections defining a series of spaced notches, said wires being retained on one side of said plate against said projections with said loop portions extending axially through said notches to the other side of said plate, and means for securing said wires against said projection.

References Cited UNITED STATES PATENTS 1,985,093 12/1934 I-Iehlgans 313-84 2,653,262 9/1953 Bowman 335-212 2,758,248 8/1956 Garrett et al. 315-27 2,833,947 5/1958 Gundert et al. 313-78 3,296,570 1/1967 Uetake et a1. 335-12 7. The mounting means of claim 6 wherein the ends of 25 JAMES LAWRENCE P r immy Examiner said loop portions are spaced such that when selectively engaged by the clips a desired geometric shape is defined.

V. LAFRANCHI, Assistant Examiner. 

1. IN A VACUUM TUBE PRODUCING A BEAM OF ELECTRONS WHICH IS DEFLECTED IN A MANNER TO FORM A DESIRED IMAGE ON A FLUORESCENT SCREEN, A DEVICE FOR MODIFYING THE GEOMETRIC SHAPE OF THE IMAGE COMPRISING: A MOUNTING PLATE FOR SURROUNDING A PORTION OF THE TUBE THROUGH WHICH THE BEAM PASSES; A PAIR OF SPACED SPRING RETAINERS ATTACHED TO SAID PLATE; A RETAINING ELEMENT ATTACHED TO AND EXTENDING BETWEEN SAID SPRING RETAINERS; A PERMANENT MAGNET CLAMPED BETWEEN SAID ELEMENT AND SAID PLATE AS URGED BY SAID SPRING RETAINERS, THE DIMENSIONAL RELATION OF SAID ELEMENT AND RETAINERS BEING SUCH AS TO PERMIT SAID MAGNET TO BE POSITIONED IN ALL DIRECTIONS IN A PLANE PARALLEL TO SAID PLATE AND TO PERMIT SAID MAGNET TO BE ROTATED ON ITS OWN AXIS. 